Plasmacytomas which produce homogeneous immunoglobulins can be induced in the BALB/c strain of mice by the intraperitoneal injection of mineral oil. By screening these immunoglobulins against various antigens groups of proteins have been identified which in fact find the same antigen. We are currently involved in structural studies involving determination of both primary amino acid sequence and three dimensional configuration of proteins with binding specificity for phosphorylcholine B-(1 yields 6)-D-galactan and levans. The Fab fragment from a phosphorylcholine binding protein, McPC603, has been crystallized. A combination of amino acid sequence and X-ray crystallographic studies have resulted in the construction of a molecular model of this protein. The interaction of hapten with the antibody combining site is currently being studied. Similar experiments have been initiated on a second crystallized protein with specificity for B-(1 yields 6)-D-galactan.